On Sat, Dec 31, 2022 at 09:26:44PM +0100, Jó Ágila Bitsch wrote: > On Sat, Dec 31, 2022 at 8:47 PM Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote: > > > > On Fri, Dec 30, 2022 at 09:11:43PM +0100, Jó Ágila Bitsch wrote: > > > > > > During device enumeration, a host recognizes that the announced > > > USB version is at least 2.01, which means, that there are BOS > > > > Where is this 2.01 value specified? I don't remember seeing it in the > > usual USBIF documents. > > This is actually from the backport of BOS descriptors to USB 2 > > Citing Randy Aull from > https://community.osr.com/discussion/comment/249742/#Comment_249742 > > There is no specification called USB 2.1, however there is such a thing as a USB 2.1 device. > > The USB 2.0 ECN for LPM introduced a new descriptor request to the enumeration process > > for USB 2 devices (the BOS descriptor set). The problem is that software can't request a new > > descriptor type to old devices that don't understand it without introducing compatibility issues > > with some devices (more than you would probably expect). So, software needed a way to > > identify whether a device could support the host requesting a BOS descriptor set. The solution > > in this ECN was to require devices supporting the ECN to set their bcdUSB to 0x0201 (2.01). > > > > Now, when we created the USB 3.0 spec, we again wanted to leverage the BOS descriptor, not > > only because we wanted to mandate USB 2 LPM in 3.0 devices when operating at high-speed, > > but also so the device can indicate to a host that it can operate at SuperSpeed (to support > > everyone's favorite "your device can perform faster" popup). Knowing that when operating at > > high-speed these devices needed to report the BOS descriptor set, we knew that it couldn't > > set the bcdUSB to 0x0200. We mistakenly wrote it down as 0x0210 instead of 0x0201 in the > > 3.0 spec (perhaps we just said "two point one" which might have been ambiguous) when we > > were trying to just be consistent with the requirement from the LPM ECN. So, rather than > > changing it back to 0x0201 in the USB 3.0 spec, we just ran with it. > > > > So, there are USB 2.0 devices, USB 2.01 devices and USB 2.1 devices, even though the latest > > revision of the USB 2 spec is USB 2.0. I have recommended that the USB-IF actually create a > > USB 2.1 specification that captures all of the various errata, ECNs, etc from over the years, but > > it hasn't happened yet. Interesting history. And now that you point it out, I do see at the end of section 3 of the USB 2.0 Link Power Management ECN: Devices that support the BOS descriptor must have a bcdUSB value of 0201H or larger. > Btw, configfs already includes these version codes to support Link > Power Management (LPM) and > the associated BOS descriptor, so I didn't add that part, I only added > webusb as a condition as to > when to send BOS descriptors. Right. > > > @@ -713,14 +714,16 @@ static int bos_desc(struct usb_composite_dev *cdev) > > > * A SuperSpeed device shall include the USB2.0 extension descriptor > > > * and shall support LPM when operating in USB2.0 HS mode. > > > */ > > > - usb_ext = cdev->req->buf + le16_to_cpu(bos->wTotalLength); > > > - bos->bNumDeviceCaps++; > > > - le16_add_cpu(&bos->wTotalLength, USB_DT_USB_EXT_CAP_SIZE); > > > - usb_ext->bLength = USB_DT_USB_EXT_CAP_SIZE; > > > - usb_ext->bDescriptorType = USB_DT_DEVICE_CAPABILITY; > > > - usb_ext->bDevCapabilityType = USB_CAP_TYPE_EXT; > > > - usb_ext->bmAttributes = cpu_to_le32(USB_LPM_SUPPORT | > > > - USB_BESL_SUPPORT | besl); > > > + if (cdev->gadget->lpm_capable) { > > > > This change doesn't seem to be related to the purpose of this patch. > > Actually, it is. > > Previously, BOS descriptors would only ever be sent if the device was > lpm_capable. > For this reason, this descriptor was previously sent unconditionally. > > With my patch in place, it could be that a device is not lpm_capable, but still > wants to send BOS descriptors to announce its WebUSB capability, > therefore I added > this condition. Okay. It would be good to explain this in the patch description. Alan Stern
